The present invention is related generally to power train systems, such as those utilized in wind turbines, and specifically to an improved wind turbine power train system configured with a compound split planetary gearing system incorporating a closed carrier flex pin system in a high torque stage together with an open carrier flex pin system in a low torque stage.
Wind turbine system architectures currently in development are aimed at producing lean (low mass, low cost) and reliable machines. One solution being pursued is a hybrid wind turbine which incorporates a combination of a simplified power train (commonly a single-stage planetary gearing system) and a mid-speed generator. To further reduce the rotating mass of this configuration, a higher ratio in the planetary gearing system may be utilized, allowing a reduced size, faster running, generator to replace the mid-speed generator. In other words, if the wind turbine rotates at a given speed of “a” rpm, and if the planetary gearing system ratio is “b”, the associated generator will rotate at a speed of (a×b) rpm. The higher the ratio “b” is, the faster the generator will rotate. As a general rule, a smaller generator that rotates faster will be lighter in mass and lower in cost, thus leading to a leaner system design. Therefore, there is an industry need to increase the step up ratio “b” for the planetary gearing system within as small a space as possible, and with as small a mass as possible.
Planetary gearing systems, such as shown in FIGS. 1A and 1B are normally comprised of a sun gear in the center, orbiting planet gears (usually but not always three in number, as shown in FIG. 1A) in mesh with the sun gear, a rotating planetary carrier (coaxial with the sun gear) which is a structural member that holds the planet gears in a fixed relative position, and a ring gear which is also coaxial with the sun gear that surrounds and meshes with all the orbiting planet gears.
Traditionally, each of the planet gears is axially supported by one or more rows of planetary bearings which are, in turn, supported on a non-rotating, but orbiting, pin that is fixed at each end to a wall of a closed planetary carrier (i.e., a carrier having two walls disposed on opposite sides of the planetary gears). This arrangement theoretically splits the input torque along a number of equal load paths corresponding to the number of planet gears, and in so doing, reduces the magnitude of the gear forces acting at each gear mesh between the sun gear, the planetary gears, and the ring gear to a correspondingly smaller number.
Gears in a planetary gearing system are normally designed as spur gears, helical gears, or as double helical gears. Regardless of which gear design is used, there are two common issues which may arise. The first is that machining tolerances necessarily create variation in clearances among all the gear meshes. This means that as torsion is applied into the gearing system, the gear mesh with the least clearance will begin supporting the load by itself, until this gear mesh deflects sufficiently so that the gear mesh with the next least clearance begins to support a portion of the load. This load shifting phenomenon will progress until the entire load is fully supported by some number of the gear meshes. In other words, some gear meshes will support more load than others. There are means for introducing flexibility into the gear meshes to restore equalization of loads in the gear meshes, one of which is the use of a floating sun gear in a three planet gear system.
The second drawback to a conventional planetary gearing system employing a closed planetary carrier having two opposite walls connected by webbing is that the applied torsion will twist the closed planetary carrier, rotationally advancing one wall of the planetary carrier carrying one end of the planetary pins ahead of the opposite wall of the planetary carrier carrying the opposite ends of the planetary pins. This rotational advancement misaligns the planetary gears with their mating sun gear and ring gear, resulting in increased wear and frictional forces at the gear meshes. In addition, the supporting planetary gear bearings are subjected to the same amount of misalignment.
When utilized in wind turbine applications, planetary gear system configurations often consist of one of several common configurations described below and shown in the associated figures:
A.—a conventional closed carrier three planet epicyclic systems with a step-up ratio equaling approximately 10:1, as exemplified by the prior art FIG. 1A;
B.—a conventional closed carrier four planet epicyclic systems with a step-up ratio equaling approximately 8:1, as exemplified by the prior art FIG. 1B;
C.—a compound planetary gearing systems with a step-up ratio equaling approximately 14:1, as exemplified by the prior art FIG. 1C;
D.—a split-compound planetary gearing systems using open-carrier planetary gear sets with flex-pins in both a low torque stage, and in a high torque stage, as exemplified by the prior art FIG. 1D; and
E.—a system which is similar to “D”, but which employs closed-carrier planetary gear sets in both the low- and high-torque stages, as exemplified by the prior art FIG. 1E.
Accordingly, it would be advantageous to provide a geared power train for use in power transmission applications, such as a wind turbine application, which is configured to maximize the effective step-up ratio between an input shaft and an output shaft within a limited space, allowing for the use of lighter-mass electrical generators and lowering the overall system costs.